1. Field of the Invention
The present invention relates to a connection method for H channels (an H.sub.0 channel and an H.sub.1 channel) in an ISDN system, and to an ISDN switch and a terminal for employing that method.
2. Related Arts
While a channel of 64 kbps is a basic B channel, 23 of these B channels and a D data channel form a 23B+D channel arrangement for ISDN transmission.
Six of 23 B channels are bundled to provide a single H.sub.0 channel, and 24 B channels are bundled to provide a single H.sub.1 channel. In this case, these H.sub.0 and H.sub.1 channels are collectively called as H channels.
Various methods have been developed for selecting an H channel. One selection method for selecting the H channel is disclosed in the Japanese Laid Open Patent Application No. Tokukai-Hei 4-144,395, and in the selection method, an H.sub.0 dedicated class or a general communication class of 64 kpbs is registered for each B channel that is a constituent of an H channel.
For H.sub.0 communication, B channels for the H.sub.0 dedicated class are acquired, and for general communication, the B channels for the general communication class are acquired.
An allowable class and a regulated class for general communication are registered for each B channel. For H.sub.0 communication, all of the B channels are acquired, and for general communication, channels other than those in the regulated class are acquired.
With this method, when an H.sub.1 communication class is set, the channels can not be used for communication with the other class, and the efficiency in the use of lines is reduced. When a class is set for shared use while communicating with the other class, a block rate for H.sub.1 communication is increased.
When all of the B channels required for formation of the H channel can not be obtained, a method is employed whereby a line connection is performed when necessary B channels are obtained using a queuing system (see the Japanese Laid Open Patent Application No. Tokukai-Hei 5-153674, for example).
According to this method, when the B channels forming the H channel are randomly used, it takes a long time for the required sequential B channels to be obtained. Further, for a tandem connection, even if the B channels required between specific switches are obtained, they can not be used for communication with another H channel until B channels that are required between other switches are obtained.
Considering that the H.sub.1 communication is to be ensured between specific switches in a network, generally, channels by which other general calls are enabled are acquired between the switches. For an instance where channels are not divided into classes, according to a technique that is disclosed in the Japanese Laid Open Patent Application Tokukai-Hei 4-144395, channels that enable a plurality of H.sub.1 communications may be used separately for general calls. A queuing time will be extended and continues until the H.sub.1 communication request is realized.
This is the same as where channels that enable H.sub.1 communication are separately used for H.sub.0 communication calls.
Further, in accordance with recent progress in data compression techniques, a purpose for which communication is employed by a user, for example, the transmission of image data can to a degree be satisfied even by a communication speed of 64 kbps.
As is described above, in a network wherein H communication (H.sub.0 or H.sub.1) via H channels, and general communication via B channels can jointly exist, the provision of channels exclusively for H communication is not economical in the viewpoint of the effective use of the network, because traffic for general transmission such as telephone or facsimile communication is higher than H communication.